This invention relates to combustion control. More particularly, this invention relates to a fail-safe gas feed and ignition sequence control apparatus and method for controlling combustion in a gas-fired appliance, such as a clothes dryer.
The prior art discloses many different forms of combustion control apparatus and methods for controlling the feed of fuel, generally, fuel oil or natural gas, and ignition of the fuel in appliances such as furnaces, water heaters, cooking ovens, clothes dryers, etc. In some instances, the temperature of the environment heated by the appliance is controlled by a thermostat. In other instances, as in the case of cooking ovens and clothes dryers, the appliance is activated by a timer.
Much of the prior art relates to combustion control apparatus of a type which includes a pilot burner for igniting fuel fed to a main burner when heat is demanded. Such type was developed when fuel was plentiful as well as cheap and energy waste was tolerable. Furthermore, electrical igniters, such as spark igniters and incandescent igniters, had not been developed to the point where they were reliable and had a long life expectancy.
Since the advent of the spark igniter and the incandescent igniter, such as the silicon carbide igniter, having reliability and long life expectancy, a shift toward a type of combustion control apparatus which includes an electrical igniter has become evident in the prior art. The shift has received added emphasis in view of fuel shortages as well as the increased expense of fuel which makes energy waste intolerable.
Of the electrical igniters, the incandescent igniter has the advantage that the circuitry which comprises the combustion control apparatus is simpler and cheaper. This is because the incandescent igniter does not require a generator means for triggering sparks required for a spark igniter but only requires a means to electrically heat the incandescent igniter.
Although the prior art mentions various safety problems and in some instances provides combustion control apparatus which to some degree is fail-safe, prior art combustion control apparatus of the type which includes an incandescent igniter by and large is not fail-safe. For example, if the incandescent igniter were defective or if the fuel were to fail to ignite, a valve would nevertheless feed fuel so as to cause the danger of fire or explosion. Furthermore, if a temporary electrical power outage were to occur or if the fuel supply were interrupted, extinguishment of the ignited fuel would occur, but fuel would again feed through the valve on restoration of the electrical power or fuel supply so as to cause the danger of fire or explosion. Additionally, in some applications where a thermostat controls temperature or where there is a safety means, such as a loading/unloading door safety switch for a clothes dryer which assures that fuel is not ignited when the door is open, the danger of fire or explosion similarly exists when the thermostat cycles on and off or the safety means is actuated and then deactuated again.